An impact rotation tool reduces the speed of the rotation force of a motor with a speed reduction mechanism and converts the decelerated rotation force to a pulsed impact torque using hydraulic pressure or hammer impacts to perform a fastening task or a loosening task (refer to Japanese Laid-Open Patent Publication No. 2012-206181). In comparison with a rotation tool that uses only a speed reduction mechanism, an impact rotation tool obtains higher torque and thus improves the workability. Impact rotation tools are widely used at construction sites and assembly plants.
The impact rotation tool generates high torque and may overtighten fasteners such as bolts or screws. However, when loosely tightening a fastener to avoid overtightening, the fastener may not be fastened with the desired strength.
To tighten a fastener with a predetermined torque, a torque sensor may be arranged on the rotation shaft of the motor to measure the torque applied to the rotation shaft. Further, Japanese Laid-Open Patent Publication No. 2005-125425 describes an impact tightening tool that computes the torque by detecting the rotation angle of a motor with a rotary encoder, differentiating the rotation angle twice to compute the angular acceleration, and multiplying the angular acceleration with the moment of inertia. When the computed torque value reaches a target torque value that is set in advance, the motor is stopped.